System for demineralizing water

ABSTRACT

Water is demineralized by passage through a water demineralization system. The water demineralization system comprises a series of at least three ion exchange resin zones. The sequential series comprises a strong acid cation (SAC) resin zone, a first anion resin zone, and a weak acid cation (WAC) resin zone. The SAC resin zone comprises a SAC resin for removing cations from the water, the first anion resin zone comprises an anion resin for removing anions from the water, and the WAC resin zone comprises a WAC resin for removing cations from water without substantially splitting any salts present in the water. Means for connecting each resin zone in the series are provided so that water can pass sequentially through the system. A method is also provided for regenerating the WAC resin from sodium and ammonium exhaustants. This method comprises contacting the WAC resin with an aqueous solution of a regenerant that is substantially devoid of sulfur and halogen groups. The regenerant is selected from the group consisting of (a) organic acids, (b) inorganic acids, (c) amine salts of (a) and (b), (d) amines, and (e) combinations thereof.

BACKGROUND

The present invention is directed to a system and method fordemineralizing water using ion exchange resins.

A power plant normally employs two types of water demineralizationsystems, namely, a condensate polishing system and a make updemineralization system. The condensate polishing system furtherdemineralizes condensate water from a steam condenser. Condensate wateris already of low solids content, e.g., normally having a totaldissolved solids content below 5 parts per million (ppm). Traditionalcondensate polishing process can produce water having a final dissolvedsolids content level of about 5 to about 50 parts per billion (ppb).Condensate water that has been polished is used, substantiallyundiluted, in a power plant operating system. By further reducing thetotal dissolved solids content of water, condensate polishing processeshelp prevent corrosion to the power plant operating system.

Ion exchange processes were initially developed for the demineralizationof water containing relatively high dissolved solids contents, i.e.,above about 20 ppm. Mixed bed systems, in which the bed is a mixture ofanion exchange and cation exchange resins, were long ago accepted asbeing advantageous for polishing condensate.

A mixed bed employed in condensate polishing processes typicallycomprises a mixture of strong base anion (SBA) and strong acid cation(SAC) exchange resins. These mixed bed systems are generally regeneratedto reuse the resins and thereby reduce their operating costs. A strongacid, e.g., sulfuric or hydrochloric acid, is used to regenerate the SACresin and caustic is used to regenerate the SBA resin.

There are problems with the use of mixed bed systems in condensatepolishing processes. One of the most significant problems is that theSBA and SAC resins cannot be totally separated. Therefore, part of theSBA resin ends up contacting either the sulfuric or hydrochloric acidused to regenerate the SAC resin and is thereby put in the (a) sulfateor bisulfate or (b) chloride form, respectively. Similarly, part of theSAC resin contacts the caustic used to regenerate the SBA resin therebyputting the SAC resin into the sodium form. When the regenerated SAC andSBA resins are returned to the mixed bed and the mixed bed is put backinto service, the SBA resins that were put in the sulfate or chlorideform and the SAC resins that were put in the sodium form leak (a)sulfate or chloride and (b) sodium, respectively, from the mixed bed.This leakage, which is in the low ppb range, increases the totaldissolved content of the effluent water and can also increase theeffluent water's corrosion potential. Both of these adverse results cancontribute to corrosion of a power plant operating system.

The mixed bed systems employed as polishers typically also suffer frompoor "kinetics". Kinetics is the rate or speed at which contaminates areadsorbed onto a resin, and thereby removed from the water, at a givenflow rate of water. Furthermore, the mixed bed systems require frequentregenerations when an influent to the mixed bed has a high sodium levelor chloride, e.g., due to a leak in a steam condenser, or a high ammonialevel due to the feed of ammonium and hydrazide into the cycle. Inaddition, current water polishing systems yield effluent water whichpossesses an unacceptably high corrosion potential. Each of thesedificiencies adversely impacts the utilities operating costs.

An additional problem that mixed bed systems suffer from is that the SACresin has some level of sulfonates constantly being leached out from theSAC resin and onto the SBA resin. Because the SBA resin's ability toadsorb sulfonates is very low and because the amount of SBA resindownstream from particular SAC resin particles varies in the mixed bedsystem, the sulfonates eventually leak into the effluent water from themixed bed. The leaked sulfonates, under the high temperatures of aboiler system, break down creating sulfates which are very corrosive toa power plant operating system.

A number of efforts have been made to alleviate these problems. However,none of the previously proposed solutions satisfactorily address allthese problems.

With respect to the make up demineralization system, this systemdemineralizes water being brought into a power plant to make up forwater lost during the power plant operating cycle. Water brought intothe power plant normally has a total dissolved solids content of above10 ppm. For example, this water can be potable water. Since less thanabout two percent of the water employed in power plant operating cycleis make up water, make up demineralization systems, because of costconsiderations, do not produce water having as low a dissolved solidscontent as water treated by condensate polishing systems.

Make up demineralization systems consist of two sections. The firstsection is for primary water treatment and the second section is forpolishing effluent water from the primary water treatment section. Eachsection can consist of one or more ion exchange resin beds or zones. Theion exchange beds employed in the primary water treatment sectiongenerally have a larger cross-sectional area and a deeper depth than theion exchange beds employed in the water polishing section. Effluentwater from the primary water treatment section of the make updemineralization system normally has a dissolved solids content of below10 ppm. The polishing section of the make up demineralization systemtraditionally produces water having a final dissolved solids contentlevel as high as 50 ppb.

Typically, sodium is the predominant mineral in the effluent water fromthe make up demineralization system. Due to operating costconsiderations, this final dissolved solids content is no longersatisfactory. Although efforts have been made to lower the finaldissolved solids content level of the treated make up water, none of thepreviously proposed solutions has satisfactorily solved the problem.

Accordingly, there is a need for a system and process for polishingcondensate water that are capable of producing water having a puritycomparable to or better than that currently obtainable but which aredevoid of the problems characteristic of current polishing systems andprocesses. These problems include (a) unacceptably high sulfonate,chloride, and/or sulfate leakage in the effluent water, (b) poorkinetics, (c) frequent resin regeneration requirements, (d) crosscontamination of resins during regeneration procedures, and (e)undesirably high corrosion potential of the effluent water. In addition,there is a need for a make up demineralization system and process fordemineralizing make up water that is capable of economically yielding aneffluent water having a sodium content of less than about 5 ppb and atotal dissolved solids content of less than about 10 ppb.

SUMMARY

The present invention satisfies these needs by providing (a) a systemand (b) method for demineralizing water as well as (c) a method forregenerating a weak acid cation resin. According to this invention, thesystem comprises (a) series of at least three ion exchange resin zonesfor removing ions from the water and (b) means for connecting each resinzone in the series so that water can pass sequentially therethrough.

More particularly, the series sequentially comprises a strong acidcation (SAC) resin zone, a first anion resin zone, and a weak acidcation (WAC) resin zone. The SAC resin zone comprises a SAC resin. SACresins are capable of removing at least about 95 percent of the cationsfrom the influent water and converting the salts in the influent waterto acids. The first anion resin zone comprises an anion resin forremoving anions (now in acid form) from water passing through thesystem. The WAC resin zone comprises a WAC resin for removing cationsfrom water (predominantly sodium from the first anion resin zoneeffluent) without substantially splitting any salts present in thewater.

When the first anion zone comprises a strong base anion (SBA) resin, theseries comprising the SAC, SBA, and WAC resin zones can serve as acondensate polisher. The presence of the WAC resin in the WAC resin zoneof the water polisher enables the condensate polisher to produceeffluent water having an acceptable corrosion potential. In additionthis configuration of resin zones (a) reduces the sulfonate leakage fromthe condensate polisher, (b) improves the system's kinetics, and (c)eliminates cross contamination of the resins during the regenerationstep. The WAC resin regeneration process of the present invention alsoreduces the chloride and sulfate leakage from the condensate polisher.

The water demineralization system of the present invention can also beemployed as a make up demineralization system to demineralize waterbrought into the power plant operating system to compensate or make upfor water lost during operation of the power plant. In a first make updemineralization system embodying features of the present invention, thefirst anion resin zone comprises an anion resin selected from the groupconsisting of weak base anion resins and mixtures of weak base anionresins and strong base anion resins. In this embodiment of the make updemineralization system, the cation and first anion resin zones areprimary water demineralization zones and the weak acid cation resin zoneis a water polishing zone.

In a second make up demineralization system embodying features of thepresent invention, the system further comprises a second anion resinzone between the SAC resin zone and the first resin zone. In thisembodiment of the make up demineralization system, the first anion resinzone comprises a SBA resin and the second anion can either comprise ananion resin selected from the group consisting of WBA resins andmixtures of WBA and SBA resins or can comprise a SBA resin. When thesecond anion resin zone comprises a resin selected from the groupconsisting of WBA resins and mixtures of WBA and SBA resins, the SACresin zone, second anion resin zone, and first anion resin zone areprimary water demineralization zones and the WAC resin zone is a waterpolishing zone. Alternatively, when the second anion resin zonecomprises a SBA resin, the SAC and second anion resin zones are primarywater demineralization zones and the first anion and WAC resin zones arewater polishing zones.

In a third make up demineralization system embodying features of thepresent invention, the first anion resin zone comprises a SBA resin andthe system further comprises a primary cation resin zone and a primaryanion resin zone. The primary cation and primary anion resin zones areprimary water demineralization zones and sequentially precede the SAC,first anion, and WAC resin zones, the latter three zones being waterpolishing zones. In this embodiment, the primary cation resin zonecomprises a cation resin for removing cations from water and the primaryanion resin zone comprises an anion resin for removing anions fromwater.

The resin zones employed in the water demineralization system of thepresent invention can be housed in either a tower or in separate vesselsor in any combination of towers and separate vessels. It is preferredthat the depth of the WAC resin in the WAC resin zone be from about 9 toabout 48 inches. Below about 9 inches the contact time for satisfactoryion exchange is too short and above about 48 inches the pressure dropacross the WAC resin zone is undesirably high.

Water passed through the condensate polisher of the present inventioncan yield an effluent having a sodium, chloride, and sulfate content ofless than about 1,000 parts per trillion (ppt) each and a conductivityof less than about 0.5 μS/cm at about 25° C. In addition, the condensatepolisher system and process of the present invention is capable ofproducing effluent water having a sodium, chloride, and sulfate contentof less than about 100 ppt each. In fact, the effluent water can have asodium, chloride, and sulfate content of less than about 50 ppt each anda conductivity of less than about 0.07 μS/cm at about 25° C.

Water passed through the make up demineralization system of the presentinvention can economically yield an effluent having a sodium, chloride,and sulfate level lower than about 2 ppb each.

As noted above, the corrosion potential of the water demineralizationsystem's effluent water can be reduced by the process of the presentinvention. This is especially important for condensate polishers. Asexplained in detail in the Description section, infra, this isaccomplished by the use of the WAC resin in the WAC resin zone. Onemethod of reducing the effluent's corrosion potential requires adjustingthe concentration of sodium in the effluent water such that it is atleast substantially equal to or greater than the combined concentrationof chloride and sulfate in the effluent. By so adjusting the ionconcentration in the effluent, the effluent is thereby maintained at asubstantially neutral to slightly alkaline pH. Water of this pHpossesses a reduced corrosion potential.

An exemplary method for manipulating the sodium content of the effluententails regenerating the WAC resin in the WAC resin zone so that theamount of sodium left on the regenerated WAC resin is less than about 5percent of the theoretical binding capacity of the WAC resin. Morepreferably, the amount of sodium left on the regenerated WAC resin isless than about 1 percent of the WAC resin's theoretical bindingcapacity.

As also noted above, the present invention is capable of reducingchloride and sulfate leakage into the effluent water. To accomplish thisaspect of the invention, the WAC resin in the WAC resin zone isregenerated with an aqueous solution of a regenerant that is (a)substantially devoid of sulfur and halogen groups and (b) selected fromthe group consisting of (i) organic acids, (ii) inorganic acids, (iii)amine salts of (i) and (ii), (iv) amines, and (v) combinations thereof.Exemplary organic acids are aliphatic carboxylic acids (e.g., aceticacid), dicarboxylic acids, hydroxy acids (e.g., citric acid), carbonicacids, carbamic acids, and ethylenediaminetetraacetic acid. Exemplaryinorganic acids are phosphoric acid, phosphorous acid, and hypo-boricacid. Exemplary amine salts include, but are not limited to, ammoniumcitrate, ammonium carbonate, and ammonium bicarbonate. Ammoniumbicarbonate can optionally be formed in situ. In addition, the ammoniumbicarbonate regenerated WAC resin can be contacted with carbonic acid,thereby putting the WAC resin in the hydrogen form. Exemplary amines aremorpholine and amino-2-methyl-2-propanol. In general, the amount ofregenerant present in the aqueous regenerating solution is less thanabout 10 weight percent of the solution.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a schematic representation of a first water demineralizationsystem embodying features of the present invention and capable of beingemployed as a condensate polisher;

FIG. 2 is a schematic representation of a second water demineralizationsystem embodying features of the present invention and capable of beingemployed as a condensate polisher;

FIG. 3 is a schematic representation of a third water demineralizationsystem embodying features of the present invention and capable of beingemployed as a make up demineralization system;

FIG. 4 is a schematic representation of a fourth water demineralizationsystem embodying features of the present invention and capable of beingemployed as a make up demineralization system; and

FIG. 5 is a schematic representation of a fifth water demineralizationsystem embodying features of the present invention and capable of beingemployed as a make up demineralization system.

DESCRIPTION

The present invention is directed to (a) a system and (b) method fordemineralizing water and (c) a method for regenerating a weak acidcation (WAC) resin. The methods and system of this invention are capableof being employed as a condensate polisher and of producing high puritywater wherein (a) the effluent water contains low sulfonate, chloride,and sulfate levels, (b) the resins, with only one exception, do notrequire frequent regenerations, (c) the resins are not crosscontaminated during the regeneration process, (d) the system exhibitsexcellent kinetics, and (e) the effluent water has a low corrosionpotential. In addition, the method and system of the present inventionare capable of being employed as a make up demineralization system andof producing effluent water having a sodium, chloride, and sulfatecontent of below about 2 pbb each.

With reference to FIG. 1, a first water demineralization system 10embodying features of the present invention, and capable of beingemployed as a condensate polisher, comprises a strong acid cation (SAC)zone 12, a strong base anion (SBA) zone 14, and a weak acid cation (WAC)zone 16. The SAC resin zone 12 comprises a SAC resin, the SBA resin zone14 comprises a SBA resin, and the WAC resin zone 16 comprises a WACresin.

The terms "strong" and "weak" refer to a resin's degree of ionization(or dissociation into ions). A strong resin is a highly ionized resin. Aweak resin is a weakly ionized resin.

The strength of acidity or basicity of an ion exchange resin can bedetermined by titration. Titration measures the change in pH value whilea suspension of the ion exchange resin is neutralized through theaddition of an alkali or an acid, as appropriate. In the case of the SACresin, the pH value starts at about 1 and, as alkali is added, increasesto about 12. In contrast, when the WAC resin is similarly neutralized,the pH value starts at about 3 and requires much more alkali to reachabout 12. When the SBA resin is neutralized with acid, the pH valuestarts at about 13 and drops under about 2. In contrast, when a weakbase anion (WBA) resin is similarly neutralized, the pH value starts atabout 8 and requires much more acid to reach about 2.

The SAC resin easily splits salts, converting them to acids. Bycontrast, the WAC resin cannot readily split salts. In addition, the WACresin operates efficiently only with waters in the pH range above about7.

The SBA resins are highly ionized and can operate over the entire pHrange. Therefore, the SBA resins can remove both the highly dissociatedstrong acids (sulfuric and hydrochloric) and the weakly dissociatedacids (carbonic and silicic). The WBA resins, on the other hand, arehighly ionized in the salt form only and operate only when the pH isbelow about 7. Being weakly ionized in the base form, the WBA resinshave little, if any, salt-splitting capacity. Accordingly, the WBAresins can remove strong acids but not weak acids. For example, WBAresins cannot remove weakly dissociated acids such as carbonic andsilicic acids. However, the capacity of the WBA resins to remove thestrong acids is much greater than that of the SBA resins.

Exemplary functional groups for WAC, SAC, SBA, and WBA resins are setforth in Table I.

                  TABLE I                                                         ______________________________________                                        Resin            Functional Groups                                            ______________________________________                                        WAC              carboxylic                                                   SAC              sulfonic                                                     SBA              quaternary ammonium                                          WBA              primary, secondary,                                                           and/or tertiary amines                                       ______________________________________                                    

Exemplary SAC resins include, but are not limited to sulfonicpolystyrene resins made by sulfonating a copolymer of styrene anddivinylbenzene. Exemplary WAC resins include, but are not necessarilylimited to, carboxylic resins. Macroporous WAC resins are preferredbecause of greater resistance to breakage from osmotic shock.

Exemplary SBA resins include, but are not necessarily limited to, Type Iand Type II resins. Type I and II resins can be further classified asstandard, porous, or macroreticular. Exemplary WBA resins include, butare not necessarily limited to, polystyrene polyamide, phenolicpolyamine, epoxy polyamine, acrylic polyamine, and macroreticulartertiary amine resins. In addition, ion exchange resins containing amixture of weak-base and strong-base groups, typically referred to as"intermediate" resins, can be used in the present invention in place ofWBA resins. Accordingly, for purposes of the present description andclaims, the term "WBA resins" includes intermediate resins.

Means 18 connect each resin zone 12, 14, and 16 so that water can passsequentially through the system 10. An exemplary connector means 18 foruse in connecting separately housed resin zones 12, 14, and 16 areconduits 20. In contrast, in a second demineralization system 22embodying features of the present invention and also being capable ofbeing employed to polish condensate, the resin zones 12, 14, and 16 arehoused in a tower 24 as shown in FIG. 2. In this second system 22, theconnector means 18 comprises plates or separators 26 which separate eachzone 12, 14, and 16 that is housed in the tower 24 into discretesections of the tower 24.

Because the SAC resin in the SAC resin zone 12 always (a) precedes and(b) is separated from the SBA resin in the SBA resin zone 14 in thecondensate polishers 10 and 22, sulfonate leakage in the effluent fromthe condensate polishers 10 and 22 is reduced. In addition, because theSAC, SBA, and WAC resins are housed separately in resin zones 12, 14,and 16, respectively, the SAC, SBA, and WAC resins are not crosscontaminated during the regeneration step. Furthermore, improvedkinetics are exhibited by the condensate polishers 10 and 22 of thepresent invention.

The depth of the WAC resin in the WAC resin zone 16 is preferably fromabout 9 inches to about 48 inches. Below about 9 inches the contact timefor satisfactory ion exchange is too short. Above about 48 inches thepressure drop across the WAC resin in the WAC resin zone 16 isundesirably high.

The depth of the resins in zones 12 and 14 is preferably from about 18to about 42 inches. Below about 18 inches the resins in zones 12 and 14tend to become exhausted too rapidly and above about 42 inches thepressure drop across the resin zones 12 and 14 is unacceptably high.

In the process of the present invention, water is sequentially passedthrough the sequential series of ion exchange resin zones. Thecondensate polisher systems 10 and 22 of FIGS. 1 and 2, respectively,preferably process water at a rate of about 20 to about 100 gallons perminute per square foot (gpmpsf) of resin zone cross sectional area.Below about a flow rate of about 20 gpmpsf, the system (a) can channeland (b) is uneconomical, and above a flow rate of about 100 gpmpsf, thecondensate water cannot be satisfactorily polished. An exemplarycondensate polishing rate is about 50 to about 70 gpmpsf. In addition,to avoid damaging any of the resins, the temperature of the influentwater to the condensate polisher systems 10 and 22 is preferably belowabout 140° F.

The effluent water from the WAC resin zone 16 of either condensatepolisher system 10 or 22 is capable of having sodium, chloride, andsulfate contents of less than about 1,000 parts per trillion (ppt) eachand a conductivity of less than about 0.15 μS/cm at about 25° C. Becauseit is desirable to minimize the amount of dissolved matter in theeffluent water, it is preferred that the effluent have a sodium,chloride, and sulfate content of less than about 100, and morepreferably less than about 50, ppt each, and that the conductivity ofthe effluent be less than about 0.07 μS/cm at about 25° C.

The condensate polisher systems 10 and 22 of the present invention alsoare capable of yielding an effluent water having a reduced corrosionpotential. This accomplishment is made possible because of the presenceof the WAC resin in the WAC resin zone 16. First, the WAC resin does notsplit salts (e.g., NaCl) and therefore does not cause the formation ofacids (e.g., HCl). Second, the corrosion potential of acidic waterincreases substantially while the corrosion potential of basic waterinitially increases at a lower rate. Accordingly, it is preferred toproduce effluent water that has a substantially neutral to slightlyalkaline pH. To achieve this result, the concentration of sodium in theeffluent water from the last resin bed 16 is maintained at a level thatis at least substantially equal to or greater than the combinedconcentration of chloride and sulfate present in the effluent water.

One way of achieving this result is to leave an amount of sodium on theregenerated WAC resin employed in the WAC resin zone 16 such that theamount of sodium leakage from the WAC resin in the WAC resin zone 16 isgreater than or equal to the combined concentration of chloride andsulfate in the effluent water. An exemplary amount of sodium left on theregenerated WAC resin is less than about five percent of the theoreticalbinding capacity of the WAC resin. In general, the amount of sodium tobe left on the WAC resin 6 in the WAC resin zone 16 is inverselyproportional to the amount of the WAC resin present in the WAC resinzone 16. Typically, the amount of such residual sodium can be less thanabout one percent of the theoretical binding capacity of the WAC resin.If the sodium content of the WAC resin is too low prior to regenerationto be capable of leaving the desired amount of sodium on the WAC resinafter the regeneration step, the sodium content on WAC resin can beadjusted upward by adding caustic to the WAC resin before regeneration.

The various resins can be regenerated in situ or can be removed toseparate regenerating stations (not shown). Removal to separateregeneration facilities provides increased efficiency and safety toplant equipment downstream since there is no danger of the regenerantbeing accidentally introduced into the cycle. A preferred technique forregenerating SAC and SBA resins is disclosed in U.S. Pat. No. 4,511,657,which is incorporated herein by reference.

An exemplary technique for cleaning the WAC resin in the WAC resin zone16 comprises the following steps. First, the WAC resin is transferredfrom the WAC resin zone 16 to a regenerating station. Next, to loosenaccumulated material, air is blown through the bottom of the WAC resinto provide agitation. The WAC resin is then backwashed in order toremove any suspended matter from the WAC resin and to classify the WACresin. This backwash procedure also fluffs the WAC resin. Dilution wateris then passed through the WAC resin and the chemical used to regeneratethe WAC resin is gradually introduced to the dilution water. Thissequential procedure safeguards against contacting the WAC resin withany concentrated regenerant.

The amount of regenerant used depends on the strength of the regenerant.In general, the amount of the regenerant employed is about a 20 percentexcess of the theoretical amount required to displace substantially allof the cations from the WAC resin. Lower amounts can be used whenincreased sodium leakage is desired. The rate at which the regenerant isintroduced into the WAC resin is dependent upon the regenerant beingemployed. In general, a flow rate of about 4 to about 8 bed volumes perhour is satisfactory. The temperature of the regenerant can be ambientor elevated. An elevated regenerant temperature of about 100° to about130° F. can be used to improve the efficiency of the regenerationprocess.

The regenerant is displaced from the WAC resin with water at the samerate employed to introduce the regenerant into the WAC resin. Typically,about one to about 1.5 bed volumes of water are used to displace theregenerant. After the displacement water has been introduced, the flowof rinse water into the WAC resin is increased to a rate of about 16 bedvolumes per hour. The increased flow rate of the rinse water is used tospeed up the regeneration process. The rinse cycle is continued until alow conductivity is achieved. The final conductivity is dependent uponthe particular WAC resin and regenerant employed. An exemplaryconductivity is less than about 10 μS/cm at about 25° C. unless thefinal regenerant includes an amine in which case the conductivity willbe less than about 100 μS/cm. After the WAC resin has been rinsed to thedesired conductivity, the WAC resin is transferred to the WAC resin zone16 and the WAC zone 16 is put back into service.

In order to minimize any halogen (e.g., chloride) and sulfate leakagefrom the WAC resin, it is preferred to employ an aqueous solution of aregenerant that is (a) substantially devoid of sulfur and halogen groupsand (b) selected from the group consisting of (i) organic acids, (ii)inorganic acids, (iii) amine salts of (i) and (ii), (iv) amines, and (v)combinations thereof. The regenerant regenerates the WAC resin primarilyfrom exhaustion due to sodium and ammonium ions. The amount ofregenerant employed in the aqueous regenerating solution is less thanabout 10 weight percent of the solution.

Exemplary organic acids include, but are not limited to, aliphaticcarboxylic acids (e.g., formic acetic, and propionic acids),dicarboxylic acids (e.g., oxalic and malonic acids), hydroxy acids(glycolic, tartaric, and citric acids), carbonic and carbamic acids, andethylenediaminetetraacetic acid. Exemplary inorganic acids include, butare not limited to, phosphoric, phosphorous, and hypo-boric acids.Exemplary amine salts include, but are not limited to, ammonium citrate,ammonium carbonate, and ammonium bicarbonate. Ammonium bicarbonate canoptionally be formed in situ with carbon dioxide and ammonium hydroxide.The ammonium bicarbonate regenerated WAC resin can be used as is or theWAC regenerated resin can be contacted with carbonic acid to convert theWAC resin back to the hydrogen form. Exemplary amines include, but arenot limited to morpholine and amino-2-methyl-2-propanol (AMP).

In addition to the embodiments of the invention discussed above, thewater demineralizer system of the present invention can also be employedas a make up demineralizer system to demineralize water added to thepower plant system to compensate or make up for water lost during thepower plant's operation. For example, FIG. 3 shows a thirddemineralization system 30 which embodies features of the presentinvention and which can serve as a make up demineralization system. Thethird demineralization system 30 comprises a primary cation resin zone32 and a primary anion resin zone 34 sequentially preceding the SAC,SBA, and WAC resin zones 12, 14, and 16, respectively. In thisconfiguration, the SAC, SBA, and WAC resin zones 12, 14, and 16,respectively, are employed to polish effluent water from the primarycation and primary anion resin zones 32 and 34, respectively, while theprimary cation and primary anion resin zones 32 and 34 are employed asprimary water demineralization zones. Resin zones used as a primarywater demineralization zone have a larger cross-sectional area thanresin zones used as water polishing zones. In addition, the depth of theresins in the primary water demineralization zones typically is deeperthan the depth of the resins in the water polishing zones. An exemplaryresin depth in the primary water demineralization zones is about threeto about eight feet.

The primary cation resin zone 32 comprises a cation resin selected fromthe group consisting of SAC resins, WAC resins, and mixtures thereof,and the primary anion resin zone 34 comprising an anion resin selectedfrom the group consisting of SBA resins, WBA resins, and mixturesthereof. In order to maximize the regeneration process, different typesof resins (strong vs. weak) are preferably regenerated differently.Accordingly, to avoid problems with respect to separating resins, it ispreferred that the cation resin employed in the primary cation resinzone 32 be either a SAC or a WAC resin but not a mixture of the two. Forthe same reason, it is preferred that the anion resin employed in theprimary anion resin zone 34 be either a SBA or a WBA resin but not amixture of the two. Furthermore, for improved kinetics, it is preferredthat the cation resin used in the primary cation resin zone 32 be a SACresin. This is because the SAC resin makes the influent to the primaryanion resin zone 34 more acidic, thereby improving the kinetics in theprimary anion resin zone 34.

As shown in FIG. 4, a fourth demineralization system 36 embodyingfeatures of the present invention, and capable of use as make updemineralizer system, sequentially comprises a primary SAC resin zone38, an anion resin zone 34, a SBA resin zone 40, and the WAC resin zone16. The primary SAC resin zone 38 comprises a SAC resin and the SBAresin zone 40 comprises a SBA resin. When the anion resin zone 34comprises an anion selected from the group consisting of WBA resins andmixtures of WBA and SBA resins, the anion resin zone 34 and the SBAresin zone 40 are primary water demineralization zones. However, whenthe anion resin zone 34 comprises a SBA resin, the anion resin zone 34is a primary water demineralization zone while the SAC resin zone 40 isa water polishing zone. In either case, the primary SAC resin zone 38 isa primary water demineralization zone and the WAC resin zone 16 is waterpolishing resin zone. The resin zones 38, 34, 40, and 16 aresequentially connected by conduits 20.

Similarly, FIG. 5 depicts a fifth system 42 which also embodies featuresof the present invention and which can also serve as a make updemineralization system. The fifth system 42 comprises the primary SACresin zone 38, a primary anion resin zone 44, and the WAC resin zone 16.The primary anion resin zone 44 comprises an anion resin selected fromthe group consisting of WBA resins and mixtures of SBA and WBA resins.The primary SAC resin zone 38 and the primary anion resin zone 44 areprimary water demineralization zones and the WAC resin zone 16 is awater polishing zone.

Typically, a power plant's demand for make up water is less than itsdemand for polished condensate. In general, tap water or water ofcomparable quality is treated by any one of make up demineralizationsystems 30, 36, or 42 at a rate of about 4 to about 15 gpmpsf in theprimary zones and at a rate of about 15 to about 25 gpmpsf in the waterpolishing zones. Water leaving the last primary zone in the make updemineralization systems 30, 36, and 42 has a total dissolved solidscontent of less than about 10 ppm. Water leaving the WAC resin zone 16of the make up demineralization systems 30, 36, and 42 has a sodium,chloride, and sulfate content of less than about 2 ppb each.

The systems and processes of the present invention are capable ofproducing water having a purity comparable to or better than thatcurrently obtainable. In addition, the configuration of the presentinvention's condensate polisher system enables the resins employed inthe system (a) to be regenerated without any cross contamination of theresins, (b) to be regenerated less frequently (for all but the SAC resinin SAC resin zone 12 which is exhausted primarily due to ammonium), and(c) to exhibit improved kinetics. Furthermore, the effluent waterproduced by the condensate polisher system of the present invention (a)possesses a low corrosion potential and (b) has a low sulfonate,chloride, and sulfate content. Also, the configuration of the presentinvention's make up demineralization system yields an effluent having asodium, chloride, and sulfate level of below about 2 ppb each.

EXAMPLES

In these examples, the ability of the present invention's aqueousregenerating solution to regenerate a sodium exhausted WAC resin isdemonstrated.

Examples 1-8 Regeneration of a WAC Resin With Different AqueousRegenerating Solutions A. Methodology

A WAC resin having a sodium (Na) capacity of about 41 meg/25 ml of WACresin was placed substantially 100% in the sodium form. The Na exhaustedWAC resin was regenerated with each aqueous regenerating solution listedin Table II at a rate of about 4 bed volumes of regenerating solutionper hour (which is equivalent to about 0.5 gpm/ft³).

The following procedure was then used to determine the efficiency ofeach aqueous regenerating solution. About 25 ml of the regenerated WACresin was placed in a drying tube. About 250 ml of an approximately twopercent (wt/wt) HCl solution was passed through the resin at a rate ofabout 3.3 ml/min. A rotameter was used to monitor the flow rate. Afterbeing treated with the HCl solution, the resin was rinsed with about 50ml of deionized water at a rate of about 3.3 ml/min. The HCl effluentand the deionized water effluent from the WAC resin were collected in a500 ml volumetric flask. The combined effluents were than diluted tovolume. The amount of sodium removed by this HCl procedure wasdetermined by atomic absorption. The results of these experiments arealso listed in Table II.

                  TABLE II                                                        ______________________________________                                                                 Remaining                                                                     Meq/25 ml                                            REGENERANT                 of Sodium                                                                      Volume/    After                                                   Concentra- 100 ml     Regenera-                              Ex.  Chemical    tion, % wt/wt                                                                            resin, ml                                                                            pH  tion                                   ______________________________________                                        1    (NH.sub.4).sub.2 CO.sub.3                                                                 2.0        1,200  8.0 0.001                                  2    NH.sub.4 HCO.sub.3                                                                        3.3        1,200  8.1 0.001                                  3    (NH.sub.4).sub.3 Citrate                                                                  2.0        1,200  7.9 0.001                                  4    (NH.sub.4).sub.3 Citrate                                                                  3.4        1,200  8.0 0.001                                  5    Citric Acid 2.7        1,200  2.1 0.0001                                 6    Citric Acid 10         300    1.6 0.0003                                 7    Acetic Acid 1          3,000  2.8 0.0003                                 8    Acetic Acid 10         300    2.2 0.001                                  ______________________________________                                    

The data set forth in Table II demonstrate that aqueous regeneratingsolutions within the scope of the present invention can effectivelyregenerate a sodium exhausted WAC resin.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, in addition to polishing condensate water, thepolisher systems of FIGS. 1 and 2 can be employed to polish any waterhaving a total dissolved solids content of less than about 10 ppm.Furthermore, it is possible to employ other primary waterdemineralization zone configurations in addition to those discussedherein. Therefore, in the spirit and scope of the appended claims shouldnot necessarily be limited to the description of the preferred versionscontained herein.

What is claimed is:
 1. A system for use in demineralizing source waterfor subsequent use in a water-using apparatus, the system comprising (a)a series of at least three ion exchange resin beds arranged insequential order comprising (i) a strong acid cation resin bedcomprising a strong acid cation resin for removing cations from thesource water passing therethrough to yield a first effluent, (ii) afirst anion resin bed comprising an anion resin for removing anions fromthe first effluent passing therethrough to yield a second effluent, and(iii) a weak acid cation resin bed comprising a weak acid cation resinfor removing cations from the second effluent passing therethroughwithout substantially splitting any salts present in the second effluentto yield a third effluent, (b) means for introducing the source waterinto the strong acid cation resin bed, (c) means for passing the firsteffluent from the strong acid cation resin bed to the first anion resinbed, (d) means for passing the second effluent from the first anionresin bed to the weak acid cation resin bed, and (e) means fordelivering the third effluent from the weak acid cation resin bed to theapparatus without passing the third effluent through any additional ionexchange resin beds.
 2. The system of claim 1 wherein the first anionresin bed comprises a strong base anion resin and the strong acidcation, first anion, and weak acid cation resin beds are water polishingbeds.
 3. The system of claim 2 wherein the series further comprises aprimary cation resin bed and a primary anion resin bed, the primarycation and primary anion resin beds sequentially preceding the waterpolishing beds, wherein the primary cation resin bed comprising a cationresin for removing cations from water and the primary anion resin bedcomprising an anion resin for removing anions from water, the primarycation and primary anion resin beds being primary water demineralizingbeds.
 4. The system of claim 1 wherein the first anion resin bedcomprises an anion resin selected from the group consisting of weak baseanion resins and mixtures of weak base anion and strong base anionresins, the cation and first anion resin beds being primary waterdemineralization beds and the weak acid cation resin bed being a waterpolishing bed.
 5. The system of claim 1 wherein the system furthercomprises a second anion resin bed between the strong acid cation bedand the first anion resin bed, the second anion resin bed comprising ananion resin for removing anions from water.
 6. The system of claim 5wherein the first anion resin bed comprises a strong base anion resinand the second anion resin bed comprises an anion resin selected fromthe group consisting of weak base anion resins and mixtures of weak baseanion and strong base anion resins, the strong acid cation, secondanion, and first anion resin beds being primary water demineralizationbeds and the weak acid cation resin bed being a water polishing bed. 7.The system of claim 5 wherein the first anion resin bed comprises astrong base anion resin and the second anion resin bed comprises astrong base anion resin, the strong acid cation and second anion resinbeds being primary water demineralization beds and the first anion andweak acid cation resin beds being water polishing beds.
 8. The system ofclaim 1 wherein the resin beds are held in a tower.
 9. The system ofclaim 1 wherein the resin beds are held in separate vessels.
 10. Thesystem of claim 1 wherein the depth of the weak acid cation resin in theweak acid cation bed is from about 9 to about 48 inches.
 11. The systemfor claim 1 further comprising means for providing an aqueous solutionof a regenerant for regenerating the weak acid cation resins and meansfor delivering the aqueous solution of the regenerant to the weak acidcation resin.
 12. The system of claim 11 wherein the regenerant isselected from the group consisting of (a) organic acids, (b) inorganicacids, (c) amine salts of (a) and (b), (d) amines, and (d) combinationsthereof, the regenerant being substantially devoid of sulfur and halogengroups.
 13. The system of claim 12 wherein the regenerant is selectedfrom the group consisting of organic acids, their amine salts, andcombinations thereof.
 14. The system of claim 13 wherein the regenerantis selected from the group consisting of aliphatic carboxylic acids,dicarboxylic acids, hydroxy acids, carbonic acids, carbamic acids,ethylenediaminetetraacidic acid, phosphoric acid, phosphorous acid,hypo-boric acid, their amine salts and combinations thereof.
 15. Thesystem of claim 14 wherein the regenerant is citric acid.
 16. The systemof claim 14 wherein the regenerant is acetic acid.
 17. The system ofclaim 14 wherein the regenerant is ammonium citrate.
 18. The system ofclaim 14 wherein the regenerant is ammonium carbonate.
 19. The system ofclaim 14 wherein the regenerant is ammonium bicarbonate.
 20. The systemof claim 19 wherein the regenerant is ammonium bicarbonate, the systemfurther comprising means for forming the ammonium bicarbonate byreacting carbon dioxide and ammonium hydroxide.
 21. The system of claim20 further comprising means for contacting the ammonium bicarbonateregenerated resin with carbonic acid to convert the ammonium bicarbonateregenerated resin into the hydrogen form.
 22. The system of claim 12wherein the regenerant is an amine.
 23. The system of claim 22 whereinthe regenerant is selected from the group consisting of morpholine,amino-2-methyl-2-propanol, and mixtures thereof.
 24. The system of claim12 wherein the regenerant is selected from the group consisting ofinorganic acids, their amine salts, and combinations thereof.
 25. Thesystem of claim 1 wherein the means for introducing the source watercomprises means for introducing source water having a total dissolvedsolids content greater than 50 ppb, the first anion resin bed comprisesa strong base anion resin; whereby the third effluent has a sodium,chloride and sulfate content of less than about 1 ppb each.
 26. Thesystem of claim 1 wherein the means for introducing the source watercomprises means for introducing source water having a total dissolvedsolids content level greater than 10 parts per million (ppm); the firstanion resin bed comprises an anion resin selected from the groupconsisting of weak base anion and strong base anion resins; whereby thesecond effluent has a dissolved content of below 10 ppm; and the thirdeffluent has a sodium, chloride, and sulfate content of less than about2 ppb each.
 27. The system of claim 1 wherein the means for introducingthe source water comprises means for introducing source water having atotal dissolved solids content level greater than 10 parts per million(ppm); the first anion resin bed comprises a strong base anion resin;and the system further comprises:(a) a second anion resin bed betweenthe strong acid cation resin bed and the first anion resin bed, whereinthe second anion resin bed comprises a second anion resin, the firsteffluent passes through the second anion resin bed to yield a secondanion resin bed effluent for introduction into the first anion resin bedto yield the second effluent; (b) means for passing the first effluentfrom the strong acid cation resin bed to the second anion resin bed; and(c) means for passing the second anion resin bed effluent from thesecond anion resin bed to the first anion resin bed.
 28. The system ofclaim 27 wherein the second anion resin is selected from the groupconsisting of weak base anion resins and mixtures of weak and strongbase anion resins; whereby the second effluent has a total dissolvedsolids content of less than 10 parts per million; and the third effluenthas a sodium, chloride, and sulfate content of less that about 2 ppbeach.
 29. The system of claim 27 wherein the second anion resin is astrong base anion resin; whereby the second anion resin bed effluent hasa total dissolved solids content of less than 10 parts per million; andthe third effluent has a sodium, chloride and sulfate content of lessthan about 2 ppb each.
 30. The system of claim 1 or 27 wherein the firstanion resin bed comprises a strong base anion resin and the systemfurther comprises:(a) a primary cation resin bed comprising a cationresin, wherein the source water passes through the primary cation resinbed to yield to a primary cation resin bed effluent; (b) a primary anionresin bed comprising an anion resin, wherein the primary cation resinbed effluent passes through the primary anion resin bed to yield aprimary anion resin bed effluent for introduction into the strong acidcation resin bed to yield the first effluent, the primary anion resinbed being between the primary cation resin bed and the strong acidcation resin bed; (c) means for passing the source water from theapparatus to the primary cation resin bed; (d) means for passing theprimary cation resin bed effluent from the primary cation resin bed tothe primary anion resin bed; and (e) means for passing the primary anionresin bed effluent from the primary anion resin bed to the strong acidcation resin bed; wherein the means for passing source water to theprimary cation resin bed comprises, means for passing source waterhaving a total dissolved solids content level greater than 10 parts permillion (ppm); whereby the primary anion resin bed effluent has a totaldissolved solids content of less than 10 parts per million; and thethird effluent has a sodium, chloride, and sulfate content of less thanabout 2 ppb each.
 31. A system for polishing water comprising thefollowing arranged in sequential order:(a) a strong acid cation resinbed comprising a strong acid cation resin for removing cations fromwater passing through the strong cation resin bed to yield a firsteffluent; (b) a first anion resin bed comprising a strong base anionresin for removing anions from water of the first effluent passingthrough the first anion resin bed to yield a second effluent; (c) a weakacid cation resin comprising a weak acid cation resin for removingcations from water of the second effluent passing through the weak acidcation resin bed without substantially splitting any salts present inthe water to yield a third effluent, the first anion resin bed beingbetween the strong acid cation resin bed and the weak acid cation resinbed; (d) means for passing the first effluent to the first anion resinbed; and (e) means for passing the second effluent to the weak acidcation resin bed.
 32. The system of claim 31 wherein the first anionresin bed further comprises a weak base anion resin.
 33. The system ofclaim 31 further comprising:(a) a second anion resin bed between thestrong acid cation resin bed and the first anion resin bed, wherein thesecond anion resin bed comprises an anion resin, the first effluentpasses through the second anion resin bed to yield a second anion resinbed effluent for introduction into the first anion resin bed to yieldthe second effluent; (b) means for passing the first effluent from thestrong acid cation resin bed to the second anion resin bed; and (c)means for passing the second anion resin bed effluent from the secondanion resin bed to the first anion resin bed.
 34. The system of claim 31or 33 further comprising:(a) a primary cation resin bed comprising acation resin, water passes through the primary cation resin bed to yielda primary cation resin bed effluent; (b) a primary anion resin bedcomprising an anion resin, the primary cation resin bed effluent passesthrough the primary anion resin bed to yield a primary anion resin bedeffluent for introduction into the strong acid cation resin bed to yieldthe first effluent, the primary anion resin bed being between theprimary cation resin bed and the strong acid cation resin bed; (c) meansfor passing the primary cation resin bed effluent from the primarycation resin bed to the primary anion resin bed; and (d) means forpassing the primary anion resin bed effluent from the primary anionresin bed to the strong acid cation resin bed.